1. Field of the Invention
The present invention generally relates to the field of fiber optic cables, in particular the present invention is directed to a new and novel method for monitoring load related stresses and factors in fiber optic cables which result in damage to the cable, and more importantly the fibers within the cable.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at very high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today""s technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Because of this development, there is a growing need to have fiber optic cables operating with high efficiency with as little signal loss as possible.
An example of a common fiber optic cable cross-section can be seen in FIG. 1. At the center of the cable is a central strength member 1. The central strength member 1 can be made from a number of different materials, such as hard plastic, glass, or a glass reinforced composite and is used as a stiffening member for the cable, as well as supporting the inner sides of the buffer tubes 2. Although FIG. 1 shows three buffer tubes 2, the quantity can increase or decrease depending on the particular application the cable is to be used for. Within each buffer tube 2 is a plurality of individual optical fibers 3. The optical fibers 3 can be configured in any number of ways. For example, within each buffer tube 2 there can be a stacked ribbon configuration (as shown in FIG. 1) where each ribbon has a plurality of individual fibers and there are a number of ribbons. Alternatively, the fibers can be configured as bundles inside the buffer tube. The configuration will greatly depend on the use and application of the cable. Finally, the outer jacket 4 provides protection to the internal components of the cable, while aiding to keep all of the components together. The outer jacket provides protection from the adverse physical elements that a cable can be exposed to during its use and installation.
The various configurations of fiber optic cables allow the transmission of a large amount of information. Unlike the previous electric wires and phone lines, fiber optics uses the transmission of light through optical fibers to transmit data and information. The use of visible and near visible light in the transmission of information can result in data transmission time intervals that are a fraction of what they would normally be if standard data transmission systems were used.
The transmission of information via fiber optics is relatively simple. First an electronic data signal is converted to visible or near visible light. The light is then transmitted through an optical fiber, which has a very pure core of silica glass surrounded by a cladding layer which is silica of a purity level less than that of the core. The difference in purity levels of these two parts of the fiber result in the core and cladding having different Indices of Refraction. This difference in their refractive indices allows the light in the core to be continually reflected back into the core as it travels along the fiber. At the end of the fiber optic cable, the light is converted to whatever form is needed.
This method to transmit data has numerous advantages, over and above the fact that the transmission speed is much faster than in standard wire and cable networks. First, the amount of signal loss over a great distance is much less than that of traditional methods. This means that a lesser amount of input power is needed. Second, optical fibers are not effected by, nor do they generate, electromagnetic fields. This is a significant problem with traditional data transmission methods, requiring significant insulation measures. The use of light practically eliminates these concerns, allowing the fibers to be very small and light weight, allowing for easier and more complex installation.
However, the use of optical fibers is not without its problems. One of the most important concerns when working with optical fibers is their sensitivity to damage during manufacture and installation. Great measures and developments have been made in attempts to protect fibers from damage during these processes. Because optical fibers are made of glass, they are very sensitive to bending or crushing stresses. Often times during manufacture or installation the individual fibers or cables are bent at angles exceeding their allowable bend radius, or are placed under very high crushing loads. Such loads and stresses severely affect the mechanical and optical performance of the fibers.
When fibers are damaged under bending or crushing stresses, the transmitted light does not properly reflect off of the cladding layer at the points of damage. This can result in a dramatic decrease in signal strength in the fiber. Moreover, an even bigger problem is that this damage can go undetected until installation is complete and the fibers are tested. The individual fibers are very small and, as stated above, are often in a cable with an outer jacket so the internal damage can not be seen. This can result in very high costs in removing damaged cables or fibers and re-installing new cables and fibers.
Therefore, there is a strong need for monitoring and visualizing several possible load factors which may be experienced during cable installation or manufacture. There are existing methods to monitor these loads but have disadvantages and drawbacks. The present devices do not provide reliable and easy-to-use means for monitoring these load related factors at a low cost. Some methods use invasive technology which is very time consuming and costly and can itself result in damage to the cable or fibers. Other non-invasive methods are also costly and notoriously unreliable.
The present invention is directed to eliminating the above problems associated with being unable to detect damaged fibers or cables until installation is complete, thus greatly reducing the overall costs involved in fiber optic cable system installation.
The present invention addresses the above problems by using pressure sensitive films or tactile films in order to detect areas on a fiber optic cable where excessive loads have been applied or experienced. In the present invention, a plurality of long strips of tactile film or pressure sensitive film are inserted, at regular intervals, throughout the fiber optic cable structure in a fashion similar to that of swellable tape. The tactile film or pressure sensitive film used can be any color-changing stress sensor which is formed in the shape of a flat strip. The present invention uses strips of tactile or pressure sensitive film of different widths which are inserted periodically throughout the cable, both circumferentially and along the length of the cable. The films are located between the optical buffer tube(s) and the outer jacket of the cable. This intermittent use decreases the overall cost and weight of the cable, over using a continuous length of tactile film. It is also desirable, in the present invention, to have the tactile or pressure sensitive film with corrugated folds along the width of the film. This corrugation provides a much higher sensitivity to loads experienced by the cables due to more xe2x80x9caggressivexe2x80x9d deformation of the film during buckling.